1. Field of the Invention
The present invention relates generally to a bolometer-type infrared sensor having a thermal isolation structure. More particularly, the invention relates to a bolometer-type infrared sensor using a resistor with a hysteresis in its thermal characteristic of resistance, and a driving method of the sensor.
2. Description of the Related Art
Conventionally, bolometer-type infrared sensors have typically used bolometer materials without hysteresis in its thermal characteristic of resistance. In recent years, Kawano created an improved bolometer-type infrared sensor having a large temperature coefficient of resistance and a hysteresis in its thermal characteristic of resistance, which is disclosed by the Japanese Non-Examined Patent Publication No. 2000-55737 published in February 2000. This sensor is explained below with reference to FIGS. 1A and 1B, and FIGS. 2 to 4.
FIGS. 1A and 1B show the structure of one pixel of the prior-art infrared sensor array, which is termed an infrared sensor below.
As shown in FIG. 1B, the sensor has a diaphragm 110 for sensing infrared rays. The diaphragm 110 comprises a thin bolometer film 105, a dielectric supporting film 103, a dielectric protecting film 106, and an infrared absorbing film 107. The supporting film 103, which is located on the inner side of the bolometer film 105, supports the film 105. The film 106, which is located to cover the bolometer film 105 on the outer side thereof, is used to protect the film 105. The infrared absorbing film 107 is used to absorb infrared rays irradiated to the diaphragm 110.
The diaphragm 110 further comprises electrodes 104 and 104xe2x80x2 at each end (the lower and upper ends in FIGS. 1A and 1B) of the bolometer film 105. The electrode 104 is connected to a wiring line 114. The electrode 104xe2x80x2 is connected to a wiring line 114xe2x80x2. on operation, a pulsed bias voltage is applied across the electrodes 104 and 104xe2x80x2 by way of the wiring lines 114 and 114xe2x80x2. Due to infrared rays 111 applied, the temperature of the bolometer film 105 changes and thus, the bolometer film 105 generates electrical resistance change. As a result, by reading out the electrical resistance change of the film 105, irradiation of the infrared rays 111 is detected through the change of voltage or current caused by the pulsed bias voltage.
The diaphragm 110 in held on two banks 116 and 116xe2x80x2 of a substrate 102 by way of two beams 112 and 112xe2x80x2, thereby forming a suspended structure. This suspended structure is to constitute a thermal isolation structure of the diaphragm 110 (i.e., the bolometer film 105) from the substrate 102.
A reflector film 101 is formed on the surface of the substrate 102 sandwiched by the banks 116 and 116xe2x80x2. A cavity or space 109 is formed between the diaphragm 110 and the reflector film 101. The distance between the film 101 and the diaphragm 110 is well adjusted in such a way that almost all the infrared rays 111 are absorbed by the infrared absorbing film 107. Due to absorption of the rays 111, the temperature of the diaphragm 110 rises and thus, the electrical resistance of the bolometer film 105 changes.
The banks 116 and 116xe2x80x2 constitute the sidewalls of the cavity 109. The diaphragm 110 is thermally isolated from the banks 116 and 116xe2x80x2 by a slit 108.
The reference numerals 113 and 113xe2x80x2 denote the roots of the beams 112 and 112xe2x80x2, respectively. The reference numerals 115 and 115xe2x80x2 denote the contacts with the wiring lines 114 and 114xe2x80x2, respectively.
FIG. 2 shows the relationship between the specific resistance "sgr" and the temperature T of the bolometer film 105 used in the prior-art infrared array sensor of FIGS. 1A and 1B. A pulsed bias voltage or current is periodically applied to the bolometer film 105, thereby repeating the temperature cycle shown in FIG. 3. In FIG. 3, tf is the frame time and tro is the read-out time. The pulsed bias voltage or current is applied during the read-out time tro. The application timing of the pulsed bias voltage or current is not shown in FIG. 3. The temperature of the bolometer film 105 is gradually risen or dropped to draw the temperature cycle of FIG. 2. In this temperature cycle, the maximum variation range of temperature is xcex94Tc, which is greater than the hysteresis range xcex94Tt of temperature (i.e., xcex94Tc  greater than xcex94Tt) The maximum variation range xcex94Tc is set by adjusting the value of the pulse width tro or voltage in such a way as to be greater than (xcex94Tt+xcex94Tmax), where xcex94Tmax is the maximum temperature change of the temperature sensing section of the bolometer film 105 caused by the possible change of the infrared rays 111.
Here, when the quantity of the irradiated infrared rays 111 is equal to the reference value, the state of the bolometer film 105 is situated at the point A (temperature: Tobj) on the temperature falling curve 150 in FIG. 2. Then, the state of the bolometer film 105 is gradually changed to go along the given temperature cycle. First, the pulsed bias voltage is applied to the film 105 to start raising its temperature. Then, the temperature of the film 105 rises without changing its physicochemical structure and as a result, the specific resistance curve (Axe2x86x92B) intersects with the temperature rising curve 151 at the point B (temperature: TB). Since xcex94T is greater than xcex94Tt, the temperature of the film 105 rises furthermore. When the temperature of the film 105 becomes higher than the temperature. TB, the temperature of the film 105 rises with changing its physicochemical structure and as a result, the state of the film 105 reaches the point C (temperature: Tc=Tobj+xcex94Tc).
Subsequently, when the application of the pulsed bias voltage is stopped and the temperature of the film 105 begins to drop, the temperature of the film 105 drops without changing its physicochemical structure and as a result, the specific resistance curve (Cxe2x86x92D) intersects with the temperature falling curve 150 at the point D (temperature: TD) Thereafter, the temperature of the film 105 drops with changing its physicochemical structure from the temperature TD to the starting temperature Tobj.
If the quantity of the infrared rays 111 from the object is decreased, the temperature of the bolometer film 105 drops by xcex94Tobj with the temperature cycle in question. Therefore, the temperature cycle curves 150 and 151 are laterally shifted to the lower side (to the left side in FIG. 2) by xcex94Tobj and as a result, the point A is shifted to the point Axe2x80x2. The point Axe2x80x2 in FIG. 2 denotes the starting point of the next temperature cycle.
In this way, by detecting the temperature shift xcex94Tobj, the quantity change of the infrared rays 111 can be known while keeping the temperature coefficient of resistance (TCR) high.
In FIG. 2, the starting point of the temperature cycle is placed on the point A, which is located on the temperature falling curve 150. However, the same result as described above is obtainable if the starting point is placed on a point that is not located on the hysteresis curve 150 and 151.
FIG. 4 shows the relationship between the specific resistance 6 and the temperature T of the bolometer film 105, where the starting point is placed on the point C that is shifted to the higher temperature side from the temperature rising curve 151. The temperature of the bolometer film 105 is dropped and risen to draw the temperature cycle of FIG. 4 In this temperature cycle, the maximum variation range of temperature is T1 to T2, which is located within the hysteresis range of TD to Tu. The pulsed bias condition (i.e., the voltage value and the pulse width) is set in such a way that xcex94Tc is greater than xcex94Tt (i.e., xcex94Tc  greater than xcex94Tt).
As shown in FIG. 4, at first, the state of the bolometer film 105 is situated at the point C (temperature: T2). Then, the state of the film 105 is gradually lowered to go along the given temperature cycle. The temperature of the film 105 drops without changing its physicochemical structure to reach the point D (temperature: TD) on the temperature falling curve 150 after crossing the temperature rising curve 151. When the temperature of the film 105 further drops, the state of the film 105 reaches the point A (temperature: TA) along the temperature falling curve 150 while changing its physicochemical structure.
In the next rising step, the state of the bolometer film 105 is gradually raised without changing its physicochemical structure to go along the given temperature cycle, thereby reaching the point B (temperature: TB) on the temperature rising curve 151. If the temperature of the bolometer material is further raised, the state of the film 105 reaches the point E (temperature: T2) on the temperature rising curve 151. Thus, the first one temperature cycle is completed. Since the second temperature cycle is started from the point E, the situation change of the film 105 is the same as described here. Accordingly, the same temperature cycle as shown in FIG. 2 is carried out in and after the second temperature cycle. This means that the irradiated infrared rays 111 can be detected in the same manner as explained above with reference to FIG. 2 if the detection operation is carried out in the second or subsequent temperature cycle.
The prior-art infrared sensor of FIGS. 1A and 1B is operable stably under the condition that the temperature change xcex94Tc is set to satisfy the relationship of xcex94Tc greater than xcex94Tt+|xcex94Tobj| and that the range of the temperature cycle is set to be within the hysteresis temperature range from TD to Tu.
With the prior-art infrared sensor of FIGS. 1A and 1B, to realize the desired temperature cycle, an electrical current is intermittently supplied to the bolometer film 105 to thereby generate Joule heat. Thus, a compact infrared sensor is realized without any particular temperature rising/falling device or apparatus. The electrical resistance is measured simultaneously with the supply of the current. These current control and resistance reading operations are performed with a specific integrated circuit device.
FIG. 3 shows an example of the temperature cycle per each frame in the prior-art infrared sensor of FIGS. 1A and 1B. The resistance is measured by detecting the applied voltage and the current flown by the same. In this case, the temperature difference xcex94Tc is expressed by the following equation (1).                               Δ          ⁢                      xe2x80x83                    ⁢                      T            C                          =                                            V              B              2                                      R              B                                ⁢                                    1                              G                th                                      ⁡                          [                              1                -                                  exp                  ⁡                                      (                                          -                                                                        τ                          ro                                                                          τ                          th                                                                                      )                                                              ]                                                          (        1        )            
In the equation (1), VB is the bias voltage applied to the bolometer film 105, RB is the electrical resistance of the film 105, Gth is the thermal conductance, Tth is the thermal time constant, and Tro is the pulse width of the pulsed bias voltage (i.e., the read-out time).
To confirm the advantage of the prior-art infrared sensors of FIGS. 1A and 1B, a plurality of the prior-art infrared sensors were arranged in a matrix array at the intervals of 50 xcexcm to thereby constitute an infrared array sensor. The temperature difference xcex94Tc was designed to be 12.9xc2x0 C. As the bolometer film 105, a vanadium oxide (VO2) film having the temperature coefficient of temperature was 10%/K was used. This VO2 film contained many oxide defects that were generated intentionally. A typical VO2 film containing no phase transition has a temperature coefficient of temperature of approximately 2%/K. The film 105 thus formed had a hysteresis range xcex94Tt of 5xc2x0 C. The temperature resolution of the sensor array thus formed was measured and as a result, the temperature resolution was 20 mK with respect to the optical system of F/1.
The prior-art infrared sensor of FIGS. 1A and 1B has the following problem.
Before explaining the problem of the prior-art sensor, the responsivity Rv (V/W) of a bolometer-type infrared sensor will be explained below. The responsivity Rv is given by the following equation (2) in the low-frequency range where the thermal time constant does not cause any problem.                               R          V                =                              αη            ⁢                          xe2x80x83                        ⁢                          V              B                                            G            th                                              (        2        )            
In the equation (2), xcex1 is the temperature coefficient of resistance of the bolometer material, xcex7 is the infrared absorption rate, VB is the bias voltage applied to the bolometer film 105, and Gth is the thermal conductance. As seen from the equation (2), the responsivity Rv increases as the bias voltage VB is increased.
With the prior-art infrared sensor of FIGS. 1A and 1B, the sensor has a high sensitivity under the condition that (1) the temperature of the diaphragm 110 is within the temperature range from TD to Tu where hysteresis occurs, and that (2) the temperature rise xcex94Tc of the diaphragm 110 due to Joule heat is greater than the sum (xcex94Tt+|xcex94Tobj|) of the hysteresis range xcex94Tt and the temperature rise |xcex94Tobj | due to the irradiation of infrared rays.
However, it is extremely difficult or almost impossible to control the temperatures TD and Tu and the hysteresis range xcex94Tt in development of bolometer materials with hysteresis- Moreover, to place the operating point in the temperature range from TD to Tu, if the read-out time tro is fixed, the bias voltage VB has its upper and lower limits, as seen from the equation (1). If the bias voltage VB has its upper limit, the responsivity Rv is unable to be equal to or greater than a specific value, as seen from the equation (2). Thus, with prior-art infrared sensor of FIGS. 1A and 1B, the sensitivity is unable to be as high as desired.
Accordingly, an object of the present invention is to provide a bolometer-type infrared sensor using a resistor with a hysteresis in its thermal characteristic of resistance that increases the sensitivity, and a driving method thereof.
Another object of the present invention is to provide a bolometer-type infrared sensor using a resistor with a hysteresis in its thermal characteristic of resistance that raises the upper limit of the bias voltage, and a driving method thereof.
Still another object of the present invention is to provide a bolometer-type infrared sensor using a resistor with a hysteresis in its thermal characteristic of resistance that expands the freedom of driving, and a driving method thereof.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the invention, a bolometer-type infrared sensor is provided, which comprises:
(a) a substrate;
(b) a diaphragm supported by the substrate with a beam in a suspended manner;
the diaphragm having a bolometer film with a hysteresis in its thermal characteristic of resistance;
(c) a first temperature controller for raising or dropping temperature of the diaphragm from its outside; and
(d) a second temperature controller for raising temperature of the diaphragm from its inside by supplying electricity to the bolometer film;
wherein the first temperature controller defines a lower-side temperature of a temperature cycle while the first and second temperature controllers define an upper-side temperature thereof;
and wherein the temperature of the diaphragm is controlled according to the temperature cycle;
and wherein a signal on the diaphragm is read out at the upper-side temperature.
With the bolometer-type infrared sensor according to the first aspect of the invention, the first temperature controller raises or drops the temperature of the diaphragm from its outside. The second temperature controller raises the temperature of the diaphragm from its inside by supplying electricity to the bolometer film. The first temperature controller defines the lower-side temperature of a temperature cycle while the first and second temperature controllers define the upper-side temperature thereof. The temperature of the diaphragm is controlled according to the temperature cycle. A signal on the diaphragm is read out at the upper-side temperature.
Accordingly, the upper limit or the bias voltage is raised and the freedom of the driving method is expanded. Thus, the sensitivity is increased.
In a preferred embodiment of the sensor according to the first aspect of the invention, the thermal characteristic of resistance of the bolometer film is divided into a low-temperature region where no hysteresis is seen, a hysteresis region where hysteresis is seen, and a high-temperature region where no hysteresis is seen. The first and second temperature controllers are operated in such a way that the lower-side temperature is placed in the low-temperature region and the upper-side temperature is placed in the hysteresis region.
In another preferred embodiment of the sensor according to the first aspect of the invention, the temperature of the diaphragm is controlled according to the temperature cycle while taking a temperature rise due to irradiated infrared rays into consideration.
In still another preferred embodiment of the sensor according to the first aspect of the invention, a Peltier element and a bias controller are additionally provided. The Peltier element is controlled by the first temperature controller and the bias controller is controlled by the second temperature controller.
According to a second aspect of the invention, a method of driving a bolometer-type infrared sensor is provided. This sensor comprising:
(a) a substrate; and
(b) a diaphragm supported by the substrate with a beam in a suspended manner;
the diaphragm having a bolometer film with a hysteresis in its thermal characteristic of resistance.
The method comprises:
(i) raising or dropping temperature of the diaphragm from its outside by a first temperature controller; and
(ii) raising temperature of the diaphragm from its inside by supplying electricity to the bolometer film by a second temperature controller;
wherein the first temperature controller defines a lower-side temperature of a temperature cycle while the first and second temperature controllers define an upper-side temperature thereof;
and wherein the temperature of the diaphragm is controlled according to the temperature cycle;
and wherein a signal of the diaphragm is read out at the upper-side temperature.
With the method of driving a bolometer-type infrared sensor according to the second aspect of the invention, because of the same reason as shown in the sensor according to the first aspect, the same advantages as those in the sensor are obtainable.
In a preferred embodiment of the method according to the second aspect of the invention, the thermal characteristic of resistance of the bolometer film is divided into a low-temperature region where no hysteresis is seen, a hysteresis region where hysteresis is seen, and a high-temperature region where no hysteresis is seen. The first and second temperature controllers are operated in such a way that the lower-side temperature is placed in the low-temperature region and the upper-side temperature is placed in the hysteresis region.
In another preferred embodiment of the method according to the second aspect of the invention, the temperature of the diaphragm is controlled according to the temperature cycle while taking a temperature rise due to irradiated infrared rays into consideration.
In still another preferred embodiment of the method according to the second aspect of the invention, a Peltier element and a bias controller are additionally provided. The Peltier element is controlled by the first temperature controller and the bias controller is controlled by the second temperature controller.
In a further preferred embodiment of the method according to the second aspect of the invention, the second temperature controller is operated by changing at least one of a pulsed bias voltage, a pulsed bias current, and a pulse width of the pulsed bias voltage or the pulsed bias current.